Vibration test-cell with axial load and in-situ microscopy

ABSTRACT

A new vibration test-cell that allows a static load to be applied simultaneously with lateral vibration coupled with in-situ microscopy that allows for the ability to open a fatigue crack up to a desired gap, as well as generate acoustic emission (AE) from vibration excitation, micro-fracture events are captured by the AE measurement while the physical observation of the crack faying surfaces is performed in-situ with an optical microscope embedded in the test cell.

This invention was made with government support under N00014-17-1-2829awarded by Office of Naval Research. The government may have certainrights in this invention.

BACKGROUND OF THE INVENTION 1) Field of the Invention

The present invention relates to a vibration test-cell that allows astatic load to be applied simultaneously with lateral vibration coupledwith in-situ microscopy that allows for the ability to open a fatiguecrack up to a desired gap, as well as generate acoustic emission (AE)from vibration excitation, micro-fracture events are captured by the AEmeasurement while the physical observation of the crack faying surfacesis performed in-situ with an optical microscope embedded in the testcell.

2) Description of Related Art

Capturing micro-fracture events from fatigue crack rubbing/clapping on atraditional tensile testing machine has proven to be difficult. Themounting of a heavy mechanical shaker to the tensile testing machineeven makes it more difficult. Accordingly, it is an object of thepresent disclosure to provide a newly designed and manufacturedvibration test-cell that can overcome these difficulties by allowing astatic load to be applied simultaneously with lateral vibration from ashaker and in-situ microscopy. This allows for the ability to open afatigue crack up to a desired gap as well as generate acoustic emission(AE) from vibration excitation. Micro-fracture events are captured bythe AE measurement while the physical observation of the crack fayingsurfaces is performed in-situ with an optical microscope embedded in thetest cell.

SUMMARY OF THE INVENTION

The above objectives are accomplished according to the present inventionby providing in a first embodiment, a vibration test cell for detectingmicro fractures. The test cell may include a mechanical load frame forretaining a specimen, a mechanical shaker, an optical microscope with acamera, and an acoustic emission measuring apparatus for measuringacoustic emissions from the specimen. Further, the mechanical load framemay retain the specimen and may apply static load to the specimen. Stillyet, the mechanical shaker may apply lateral vibration to the specimenmounted on the mechanical load frame. Again, the specimen may be placedunder axial load while lateral vibration may also be applied to thespecimen. Still again, a micro fracture event may produce acoustic wavesthat may propagate in the specimen and may be captured by the acousticemission measuring apparatus. Moreover, load may be applied to open amicro fracture to a first specific displacement. Still again, acousticwaves may be generated at the first specific displacement to study crackbehavior at the first specific dis-placement. Further again, the microfracture may be opened to a second specific displacement that differsfrom the first specific displacement. Still yet, acoustic sensors may beaffixed to the specimen. Again yet, a frequency response of the specimenmay be measured at various axial loads. Further still, the mechanicalload frame, mechanical shaker, optical microscope, and/or acousticemission measuring apparatus may be used independently from one anotherto test the specimen. Again further, the mechanical load frame,mechanical shaker, optical microscope, and/or acoustic emissionmeasuring apparatus may be used simultaneously to test the specimen.

In a further embodiment, a method for determining if a specimen containsa micro-fracture event is provided. The method may include securing aspecimen to be tested to a mechanical load frame, applying an axial loadto the specimen, applying a vibration excitement to the specimen,capturing at least one visual image of the specimen, and capturing atleast one acoustic emission from the specimen. Further, an acousticsensor may be bonded to the specimen. Still yet, a micro fracture eventmay produce acoustic waves that propagate in the specimen and arecaptured. Again, axial load may be applied to open a micro fracture to afirst specific displacement. Still again, acoustic waves may begenerated at the first specific displacement to study crack behavior atthe first specific displacement. Further still, the micro fracture maybe opened to a second specific displacement that differs from the firstspecific displacement. Further yet, a frequency response of the specimenmay be measured at various axial loads. Again still, axial load andvibration excitement may be applied independently from one another totest the specimen. Still further, axial load and vibration excitementmay be applied simultaneously to test the specimen.

BRIEF DESCRIPTION OF THE DRAWINGS

The construction designed to carry out the invention will hereinafter bedescribed, together with other features thereof. The invention will bemore readily understood from a reading of the following specificationand by reference to the accompanying drawings forming a part thereof,wherein an example of the invention is shown and wherein:

FIG. 1 shows a CAD model of a test cell, isometric view, per the currentdisclosure.

FIG. 2 shows a CAD model of a test cell, side view, per the currentdisclosure.

FIG. 3 shows a picture of a test cell manufactured per the currentdisclosure with microscope-camera attachments.

FIG. 4 shows a schematic AE test under shaker vibration with axialtension.

FIG. 5 shows graphs of crack opening measurement at various axial loads.

FIG. 6 shows microscopic images of crack opening at 0 kN and 10.5 kNload.

FIG. 7 shows frequency responses of a free specimen, and the specimen at0, 5, and 10 kN axial load.

FIG. 8 shows acoustic emission (AE) hits measurement @5 kN axial load.The AE hits occurred near two resonances (535 Hz and 610 Hz).

FIG. 9 shows a typical AE signal and its frequency spectrum recordingduring crack vibration.

It will be understood by those skilled in the art that one or moreaspects of this invention can meet certain objectives, while one or moreother aspects can meet certain other objectives. Each objective may notapply equally, in all its respects, to every aspect of this invention.As such, the preceding objects can be viewed in the alternative withrespect to any one aspect of this invention. These and other objects andfeatures of the invention will become more fully apparent when thefollowing detailed description is read in conjunction with theaccompanying figures and examples. However, it is to be understood thatboth the foregoing summary of the invention and the following detaileddescription are of a preferred embodiment and not restrictive of theinvention or other alternate embodiments of the invention. Inparticular, while the invention is described herein with reference to anumber of specific embodiments, it will be appreciated that thedescription is illustrative of the invention and is not constructed aslimiting of the invention. Various modifications and applications mayoccur to those who are skilled in the art, without departing from thespirit and the scope of the invention, as described by the appendedclaims. Likewise, other objects, features, benefits and advantages ofthe present invention will be apparent from this summary and certainembodiments described below, and will be readily apparent to thoseskilled in the art. Such objects, features, benefits and advantages willbe apparent from the above in conjunction with the accompanyingexamples, data, figures and all reasonable inferences to be drawntherefrom, alone or with consideration of the references incorporatedherein.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to the drawings, the invention will now be described inmore detail. Unless defined otherwise, all technical and scientificterms used herein have the same meaning as commonly understood to one ofordinary skill in the art to which the presently disclosed subjectmatter belongs. Although any methods, devices, and materials similar orequivalent to those described herein can be used in the practice ortesting of the presently disclosed subject matter, representativemethods, devices, and materials are herein described.

Unless specifically stated, terms and phrases used in this document, andvariations thereof, unless otherwise expressly stated, should beconstrued as open ended as opposed to limiting. Likewise, a group ofitems linked with the conjunction “and” should not be read as requiringthat each and every one of those items be present in the grouping, butrather should be read as “and/or” unless expressly stated otherwise.Similarly, a group of items linked with the conjunction “or” should notbe read as requiring mutual exclusivity among that group, but rathershould also be read as “and/or” unless expressly stated otherwise.

Furthermore, although items, elements or components of the disclosuremay be described or claimed in the singular, the plural is contemplatedto be within the scope thereof unless limitation to the singular isexplicitly stated. The presence of broadening words and phrases such as“one or more,” “at least,” “but not limited to” or other like phrases insome instances shall not be read to mean that the narrower case isintended or required in instances where such broadening phrases may beabsent.

Throughout this application, various embodiments of this invention maybe presented in a range format. It should be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible subranges as well asindividual numerical values within that range. For example, descriptionof a range such as from 1 to 6 should be considered to have specificallydisclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numberswithin that range, for example, 1, 2, 3, 4, 5, and 6. This appliesregardless of the breadth of the range.

Whenever a numerical range is indicated herein, it is meant to includeany cited numeral (fractional or integral) within the indicated range.The phrases “ranging/ranges between” a first indicate number and asecond indicate number and “ranging/ranges from” a first indicate number“to” a second indicate number are used herein inter-changeably and aremeant to include the first and second indicated numbers and all thefractional and integral numerals therebetween.

Understanding the behavior of a fatigue crack can prevent aircraftstructural failure and save the lives of millions of passengerstravelling everyday around the world. The test-cell disclosed hereinenables understanding micro-fracture events during fatigue crackvibration. The test-cell was designed and manufactured in the Laboratoryof Active Materials and Smart Structures (LAMSS), USC, Columbia. Thetest-cell is inexpensive and has and a simple design. It has foursubsystems: mechanical load frame, mechanical shaker, opticalmicroscope, and acoustic emission measurement. Each of the sub-systemscan function independently or simultaneously with each other which makeit very versatile and useful.

A test-cell was designed based on a design load of 20 kN and a 3D CADmodel was generated as illustrated in FIGS. 1 and 2 . Every component inthis test cell was designed by using the concept of solid mechanics tosustain the design load with a safety margin of 1.2.

FIG. 1 shows a model device of the current disclosure wherein element: 1represents a specimen sample; 2 represents a specimen retainer; 3 is aspecimen slider with rollers; 4 is a turning plate; 5 is a turnbuckle; 6is a loading bar; 7 is a spring plate; 8 is extension springs; 9 is amechanical shaker; 10 is an optical microscope; 11 is a mountain pole;12 is an x-y plane slider; 13 is a 80/20 framing system; and 14 is at-slotted table.

The entire vibration test-cell system consists of four main sub-systems:(1) the mechanical load frame, (2) mechanical shaker, (3) opticalmicroscope, and (4) acoustic emission measurement. Each sub-system hasits own functionality and can work independently and/or coupled witheach other. Thus, each of the sub-systems can function independently orsimultaneously with each other which makes it very versatile and useful.The mechanical load frame retains the specimen firmly and appliesvarious static load. The mechanical shaker applies lateral vibration tothe specimen mounted in the load frame. The optical microscope-cameraobserves the physical changes of the micro fracture/crack in thespecimen, measures the crack opening at various load levels, andcaptures the image of the crack opening using a DSLR camera mounted inthe microscope. The acoustic emission measurement device measures theacoustic emissions during the crack rubbing/clapping due tomicro-fracture event occurring in the crack faying surface.

As illustrated in FIG. 1 , a specimen 1 is clamped by specimen retainer2 on one end of the mechanical load frame. The other end of the specimenis rested on specimen slider with rollers 3 which allows moving of thespecimen in the axial direction only. Specimen 1 is clamped betweenloading plates 4, which may be two, three, four or more loading platesand the current disclosure should not be considered limited to only twoloading plates as shown in FIGS. 1 and 2 , which is attached toturnbuckle 5. Loading bar 6 is used to turn turnbuckle 5, whicheventually applies the axial load to specimen 1. The other end ofturnbuckle 5 is connected to spring plate 7. Spring plate 7 is connectedto a plurality of extension springs, while seven (7) springs are shownin FIG. 1 , the current disclosure is not so limited and more or lesssprings are considered within the scope of the disclosure such as 3, 4,5, 8, 9, 10 springs, etc. Each spring may have a spring constant rangingfrom 50 to 140 N/mm, 70 to 130 N/mm, 90 to 120 N/mm, 100 to 115 N/mm,which in one preferred embodiment may be 110 N/mm for each spring). Thesprings are connected to a retainer wall with counter forts. Underneathspecimen 1, shaker 9 can be placed to produce vibration excitement tospecimen 1. A microscope with a DSLR camera 10 can be placed on top ofspecimen 1 hanging from mounting pole 11. An x-y slider 12 allows forfine movement of microscope/camera 10. The microscope assembly allowsmicroscope 10 to move in the X, Y and Z directions for fine adjustmentof focusing on a desired area of specimen 1. All mechanical componentsmay be placed onto frame 13, which may be constructed from 80/20aluminum framing, and secured to t-slotted table 14.

Vibration test cell 17, see FIG. 3 , was manufactured by using themachine shop facilities of the Mechanical Engineering Department,University of South Carolina. The manufactured test-cell withmicroscope-camera attachments is shown in FIG. 3 . Monitor 15 shows amagnified view of micro-fracture/crack 16 in specimen 1. Specimen 1 wasmade of aluminum A1-2024 T3. A 1-mm hole was drilled at the center ofspecimen 1 to initiate crack 16 under cyclic fatigue loading (2.3 kN to23 kN) via a hydraulic MTS machine. While fatigue crack 16 grew up to15-mm, then the fatigue-cracked specimen 1 was tested in vibrationtest-cell 17.

The acoustic emission (AE) measurement setup 200 is illustrated in FIG.4 . While specimen 1 was under axial load, mechanical shaker 9 wasplaced underneath and coupled to specimen 1 to apply the lateralvibration. Function generator 202 sends a signal 204 to amplifier 206and then on to shaker 9 to produce a vibration excitement in specimen 1.Piezoelectric Wafer Active Sensors (PWAS) 208 bonded to specimen 1receive the AE events produced by the movement, such asrubbing/clapping, of fatigue crack 17 faying surfaces due to the appliedvibration to specimen 1. The micro-fracture events produce movement,such as rubbing/clapping, of the fatigue crack 17 faying surfaces. Thisin turn produces acoustic waves that propagate in specimen 1 and arecaptured by PWAS 208. PWAS 208 converts the strain waves into electricalsignals which are amplified using at least one pre-amplifier 210 arethen recorded by the AE measurement instrumentation 212.

With this test-cell, it is possible to apply an axial load to thespecimen to open the fatigue crack to a selected displacement, such as aspecified crack gap distance between the faying surfaces, in order tofurther study the fatigue crack. The current disclosure test-cell mayalso further open a crack to greater displacements after a previousselected displacement has been studied. The specific displacement mayvary based on the specimen and testing required. Sample crack openingdisplacement results are illustrated in FIG. 5 . The microscopic-cameraimages at two load levels (0 and 10.5 kN) can be seen in FIG. 6 . Theimage processing provides the crack opening displacements as marked inthe two images in FIG. 6 .

With this test-cell, it is also possible to measure the frequencyresponse of the specimen at various axial loads. The frequency responseresults for a free specimen, and the specimen at 0, 5, and 10 kN areillustrated in FIG. 7 .

The test-cell also allows acoustic emission measurement at various axialload levels. The AE measurement results at 5 kN axial load is shown inFIG. 8 . It was found that the AE signals triggering is related to thefrequency response of the specimen. We also found that not allresonances are sensitive to produce AE signals. Certain resonances (e.g.535 Hz and 610 Hz) can trigger AE signals since these resonances canexcite the crack faying surfaces.

A typical AE signal and its frequency spectrum is shown in FIG. 9 . Itshows that the AE signal contains higher frequency content such as 40kHz, 48 kHz even though the excitation was just only 535 Hz. Hence,these AE signals are not coming from the direct shaker vibration butcoming from the crack rubbing/clapping related AE events.

This test cell easily incorporates all the systems needed for properexperimentation of a fatigue-cracked specimen. The concerns of machinevibration, mounting constraints, alignments, and positioningdifficulties are eliminated by this setup.

The largest novelty of this disclosure is the ability to apply lateralvibration under various axial load while observing the crack openingwith an in-situ microscope and acoustic emission measurement from themicro-fracture events.

Attempts in the past have been made to monitor fatigue crack growth andcrack opening displacement under load in a controlled setting (dePannemaecker, Fouvry, Buffiere, & Brochu, 2018; Noraphaiphipaksa,Manonukul, Kanchanomai, & Mutoh, 2016; Robert, Dariusz, & Kotyk, 2016;Sullivan & Crooker, 1977; Sutton, McNeill, Helm, & Boone, 2002; Suzuki &Iwanaga, 2009; Vormwald, Hos, Freire, Gonzales, & Diaz, 2018). Thisincludes digital image correlation, use of crack gauge, and finiteelement predictions, etc. The AE measurement from the crack growth hasbeen reported by many authors (Bhuiyan & Giurgiutiu, 2017; Lee et al.,2006; Ning, Chu, & Ren, 2014; Roberts & Talebzadeh, 2003). However, theAE measurement from crack rubbing/clapping under the vibrationexcitation has not been reported so far.

We also performed microscopy in metrology labs as well as mounting ahigh-resolution camera with extension tubes, and later a microscope, toa platform on an MTS machine. These techniques have led to less thandesirable results. Viewing the specimen under a microscope in themetrology lab was inconvenient and did not allow for observation of aloaded specimen. Using the high-resolution camera was convenient and thespecimen could be viewed while loaded but at a limited magnification.The addition of extension tubes made for higher magnification but finetuning to the location of crack growth was difficult and minutevibrations caused by the MTS machine made the images captures unusabledue to the low weight of the camera and decreased the field of view. Amicroscope was then used but experienced similar issues to the camerabut because the microscope was much larger, the viewing angle was notperpendicular to the specimen due to limited space between the mountinglocation and specimen.

The benefits of the current disclosure are several fold:

-   -   This test-cell can easily incorporate four sub-systems        (mechanical load frame, mechanical shaker, optical microscope        with digital single-lens reflex camera (DSLR) camera, and        acoustic emission measurement) independently and/or coupled with        each other;    -   The test-cell load frame is simple in design and easy to        manufacture;    -   The test-cell is significantly cheaper than its commercial        counterparts such as tensile testing machine;    -   The operation of the test-cell is so easy to operate using human        power and does not involve complicated hydraulics and electric        power requirement;    -   This test-cell load frame is fully-mechanical and capable of        applying up to 20 kN axial load;    -   This test-cell can measure the crack opening displacement at        various axial load levels;    -   This test-cell contains a x-y-z motion slider for microscope        camera for fine focus adjustment;    -   This test-cell can be used to generate crack rubbing and        clapping of the crack-faying surfaces at various axial loads;    -   This test-cell allows the acoustic emission measurement for        crack rubbing/clapping due to specimen vibration;    -   This test-cell can be used to measure the frequency response at        various axial static loads;    -   This test-cell allows the study of fatigue crack behavior under        various axial load; and    -   This test-cell can be used to achieve a specific crack opening        displacement and study the fatigue crack behavior at this        displacement and/or the crack may be opened to another specific        crack opening displacement, which differs from the first        specific crack opening displacement.

All patents, patent applications, published applications, andpublications, databases, websites and other published materials referredto throughout the entire disclosure herein, unless noted otherwise, areincorporated herein by reference in their entirety.

While the present subject matter has been described in detail withrespect to specific exemplary embodiments and methods thereof, it willbe appreciated that those skilled in the art, upon attaining anunderstanding of the foregoing may readily produce alterations to,variations of, and equivalents to such embodiments. Accordingly, thescope of the present disclosure is by way of example rather than by wayof limitation, and the subject disclosure does not preclude inclusion ofsuch modifications, variations and/or additions to the present subjectmatter as would be readily apparent to one of ordinary skill in the artusing the teachings disclosed herein.

What is claimed is:
 1. A vibration test cell for detecting micro fractures comprising: a mechanical load frame for retaining a specimen; a mechanical shaker; an optical microscope with a camera; an acoustic emission measuring apparatus for measuring acoustic emissions from the specimen; and wherein a frequency response of the specimen is measured at various axial loads.
 2. The vibration test cell of claim 1, wherein the mechanical load frame retains the specimen and applies static load to the specimen.
 3. The vibration test cell of claim 1, wherein the mechanical shaker applies lateral vibration to the specimen mounted on the mechanical load frame.
 4. The vibration test cell of claim 1, wherein the specimen is placed under axial load with lateral vibration also applied to the specimen.
 5. The vibration test cell of claim 1, wherein a micro fracture event produces acoustic waves that propagate in the specimen and are captured by the acoustic emission measuring apparatus.
 6. The vibration test cell of claim 1, wherein axial load is applied to open a micro fracture to a first specific displacement.
 7. The vibration test cell of claim 6, wherein acoustic waves are generated at the first specific displacement to study crack behavior at the first specific displacement.
 8. The vibration test cell of claim 7, wherein the micro fracture is opened to a second specific displacement that differs from the first specific displacement.
 9. The vibration test cell of claim 1, wherein acoustic sensors are affixed to the specimen.
 10. The vibration test cell of claim 1, wherein the mechanical load frame, mechanical shaker, optical microscope, and/or acoustic emission measuring apparatus are used independently from one another to test the specimen.
 11. The vibration test cell of claim 1, wherein the mechanical load frame, mechanical shaker, optical microscope, and/or acoustic emission measuring apparatus are used simultaneously to test the specimen.
 12. A method for determining if a specimen contains a micro-fracture event comprising: securing a specimen to be tested to a mechanical load frame; applying an axial load to the specimen; applying a vibration excitement to the specimen; capturing at least one visual image of the specimen; and capturing at least one acoustic emission from the specimen; and wherein axial load is applied to open a micro fracture to a first specific displacement.
 13. The method of claim 12, wherein a micro fracture event produces acoustic waves that propagate in the specimen and are captured.
 14. The method of claim 12, wherein acoustic waves are generated at the first specific displacement to study crack behavior at the first specific displacement.
 15. The method of claim 14, wherein the micro fracture is opened to a second specific displacement that differs from the first specific displacement.
 16. The method of claim 12, wherein a frequency response of the specimen is measured at various axial loads.
 17. The method of claim 12, wherein axial load and vibration excitement are applied independently from one another to test the specimen.
 18. The method of claim 12, wherein axial load and vibration excitement are applied simultaneously to test the specimen. 